1. Field of the Invention
The present invention relates to the field of frequency synchronization in multicarrier communication systems, for example those which, for synchronization purposes, utilize pilot subcarriers with increased power. The invention is particularly, but not exclusively, concerned with the tuning of receivers for OFDM signals, for example digital video broadcast receivers.
2. Description of the Related Art
A multicarrier communication system utilizes a large number of equally spaced subcarriers for data transmission and for other auxiliary functions. In order to perform the demodulation process correctly, the receiver developed for such a system must be frequency-locked with a very small residual frequency offset. To facilitate frequency locking, a class of multi carrier communication systems employs a set of pilots transmitted at selected subcarriers with an increased power. These selected subcarriers should form a pilot insertion pattern with optimal autocorrelation characterized by low sidelobe values.
FIG. 1 shows an example of the power associated with the DFT coefficients reconstructed by a receiver in an ideal case when there is no noise or interference and the communication channel is distortionless. However, in practical applications the received signals are corrupted by wideband noise as well as by strong narrowband signals generated at some frequencies by various interfering sources. Furthermore, when the transfer function of the communication channel has not been corrected, the channel itself will introduce both magnitude and phase distortions. FIG. 2 illustrates the distortions resulting from a combined effect of frequency-selective fading, noise and strong interference occurring at some subcarrier frequencies. As seen, in this case discrimination between pilot subcarriers and data subcarriers is much more difficult which will result in a degraded performance of any system used for frequency locking.
FIG. 3 shows the two components of the total frequency offset xcex94f comprising the fractional part xcex5xcex94fc and the coarse frequency offset Jxcex94fc where J is 2 in the illustrated example, and where xcex94fc is the subcarrier separation. Several time-domain methods are available for estimating and correcting the fractional frequency offset in a multi carrier communication system. However, the majority of methods proposed for estimating the coarse frequency offset are based on coherent processing which utilizes known phase relationships between signals to be discriminated. Such methods are not well suited for initiating the frequency acquisition process when the channel transfer function has not yet been corrected.
Aspects of the invention are set out in the accompanying claims.
The preferred embodiment of the invention enables the estimation of a coarse frequency offset by suitably processing received signals which may have been distorted severely by the uncorrected communication channel and may have been corrupted by wideband noise and strong narrowband interference.
In accordance with the preferred embodiment, a signal processing apparatus determines a frequency offset for tuning purposes by obtaining, for each of a plurality of candidate frequency offsets, the sum of the powers of a set of subcarriers. If certain conditions are met, the frequency offset associated with the largest of these sums is used for adjusting tuning.
Preferably, for each candidate frequency offset, the power sum excludes the U largest of the subcarrier powers, where U is an integer of one or more. It has been found that this avoids erroneous selection of a frequency offset as a result of strong interfering signals.
Preferably, a frequency offset is selected for tuning purposes only if the associated power sum bears a predetermined relationship with at least some of the power sums associated with the other frequency offsets. Preferably, the largest sum must bear a predetermined relationship with the average of the next L largest power sums, where L is an integer of one or more.
Preferably, the power sum associated with each frequency offset is integrated over a plurality of symbol periods, for more reliable determination of the desired frequency offset. Preferably, this operation ceases when a predetermined criterion is met, this criterion indicating with a high degree of reliability that the correct frequency offset has been determined. This means that the number of symbol periods needed for reliable frequency offset estimation is not fixed in advance, thus resulting in as short a lock-in period as possible.
To allow for variable observation periods, the criterion which establishes that the desired frequency offset has been determined is a function of the number of symbol periods used for observation.
The present invention can thus provide a signal processing apparatus which utilizes a sequential decision procedure to minimize the time required for determining a reliable estimate of a coarse frequency offset.